Internal combustion engines and gensets: Science and limits
Why squeezing harder gets more work from each drop
What this tab covers What physically happens in the conversion, step by step, the science behind it, and the hard limits that physics sets on how well it can ever work.A piston engine burns fuel inside a closed cylinder; the hot, high-pressure gas pushes a piston, and a crankshaft turns that push into rotation for wheels, propellers or a generator. Car petrol engines typically convert about a quarter of their fuel to work, while the best large gas and diesel engines exceed 50%.
The conversion, step by step
Combustion and expansion on the pistonChemical (petrol, diesel or gas) → Heat and pressure, then mechanical (crankshaft) · about 28-32% of fuel energy leaves a typical petrol car engine as work in mixed driving (calculated from DOE's 68-72% engine losses), with 16-25% reaching the wheels; 40-41% best car engines at peak; 46-48% leading truck diesels; 53.09% record heavy-duty diesel129Generator (in a genset)
Mechanical → Electrical · up to 98.9% for large power-plant generators; small alternators are lower8Whole gas-engine power plant
Chemical (gas) → Electrical · 50.2% (Wärtsilä 50SG, 50 Hz)3
The science
Engines follow two idealised cycles. In the Otto cycle (petrol and most gas engines) a fuel-air mix is compressed and lit by a spark. In the Diesel cycle only air is compressed, so hot that injected fuel ignites by itself; this allows higher compression and better efficiency. Two-stroke engines fire on every turn; four-stroke engines on every second turn. Efficiency rises with compression ratio, with lean mixtures, and with running at steady, high load. Losses are heat into the cooling water and cylinder walls, hot exhaust, friction and pumping air in and out. Gensets couple the engine to an alternator running at a fixed speed so the electricity keeps grid frequency. Lean-burn gas engines for power plants and the slow two-stroke engines in ships are the most efficient piston engines; small car engines, which mostly run at light load, are the least.
Limits set by science
| Limit | Value | Why it exists | Best today |
|---|---|---|---|
| Ideal Otto cycle | about 64-65% at compression ratios of 13-14 (calculated: 1 - r^(-0.4) for air); about 55% at r = 14 with a more realistic exponent of 0.3 (calculated)2 | Higher compression gives more work per unit of heat, but petrol knocks (self-ignites) if compressed too far, which caps the ratio. | Toyota's 2.0-litre engines reach 40% (13:1) and 41% (hybrid, 14:1). |
| Ideal Diesel cycle and the record | about 63% for an ideal air-standard Diesel cycle at compression ratio 18 and cut-off ratio 2 (calculated: 1 - r^(-0.4) x (2^1.4 - 1) / (1.4 x (2 - 1)))1 | Diesels compress air only, so they avoid knock and can use high compression; heat loss to walls, exhaust heat and friction remain. | Weichai's 53.09% engine (2024), up from 50% in 2020 and 52% in 2022, against 46-48% for leading US and European engines. |
| Large gas engines | about 50% electrical3 | Big, slow cylinders (500 mm bore) lose less heat per unit of fuel. | Wärtsilä 50SG: 50.2% at 50 Hz, 50.0% at 60 Hz, about 18.4-18.9 MW per engine. |
Sources
- Weichai unveils diesel engine with 53.09% brake thermal efficiency DieselNet, 2024
- Toyota Unveils More New Gasoline ICEs with 40% Thermal Efficiency SAE Mobility Engineering, 2018
- Wärtsilä 50SG gas engine power plant Wärtsilä, 2025
- Air-cooled generators (GEN-A) GE Vernova, 2025
- FOTW #1044: 12-30% of energy put into a conventional car is used to move the car down the road US Department of Energy, Vehicle Technologies Office, 2018